Non-linear interference is generated when at least one offending signal experiences some non-linear behavior. This non-linear behavior could be experienced anywhere the offending signals are present. The non-linear behavior could be in a radio, signal transmission line network, antenna system, or in the radio frequency (RF) environment where the offending signals are transmitted. One of the problems associated with non-linear interference is when the interference couples into a receiver and the interference overlaps an assigned frequency channel for the receiver. The interference will degrade the quality of the received signal in the receiver, thereby degrading performance.
In a cellular base station, this non-linear behavior can be attributed to the active components in the transmitter (such as the power amplifier), active components in the receiver (such as the low noise amplifier (LNA) or a frequency converter), or a passive device that exhibits passive intermodulation (PIM). The PIM sources could occur in many places. Some examples are in the radio's filter, connectors, cable assembly from the radio to the antenna, in multiplexers if multiple radios are multiplexed onto the same cable or antenna, in the antenna, or in the environment external to the antenna.
Non-linear interference generates interference at multiple frequencies. This interference can be problematic when some of the generated interference falls into an assigned receive channel of the base station. The interference can desensitize the receiver, thereby reducing performance of the receiver. To understand how non-linear interference is generated at multiple frequencies it is useful to model the non-linear behavior with a simple Taylor series with 1st (linear) and 3rd (non-linear) order terms. This model is:yout(t)=c1xin(t)+c3xin3(t)When the input signal consists of 2 modulated RF carriers, then the input signal can be expressed asxin(t)=A1(t)cos[2πf1t+φ1(t)]+A2(t)cos[2πf2t+φ2(t)]The first RF carrier in this expression has an amplitude modulation denoted by A1(t), a phase modulation denoted by φ1(t), and an RF carrier frequency of f1.The output signal has the following 8 frequency components:yout(t)=[c1A1(t)+¾c3A13(t)+c3A1(t)A22(t)]cos[2πf1t+φ1(t)]+[c1A2(t)+¾c3A23(t)+c3A12(t)A2(t)]cos [2πf2t+φ2(t)]+½c3A12(t)A2(t)cos[2π(2f1−f2)t+2φ1(t)−φ2(t)]+½c3A1(t)A22(t)cos[2π(2f2−f1)t+2φ2(t)−φ1(t)]+½c3A12(t)A2(t)cos[2π(2f1+f2)t+2φ1(t)+φ2(t)]+½c3A1(t)A22(t)cos[2π(2f2+f1)t+2φ2(t)+φ1(t)]+¼c3A13(t)cos[2π3f1t+3φ1(t)]+¼c3A23(t)cos[2π3f2t+3φ2(t)]
Of all these terms, the only linear terms are those that have A1(t) or A2(t) as their only amplitude modulation terms. The rest are non-linear interference terms, and can cause problems if any of them couple into a receiver that is operating in a frequency channel that overlaps with a non-linear interference term. Real non-linear behavior can also exhibit even-order non-linear terms, and terms with higher than 3rd order. The input signal in the above equation has 2 modulated carriers. However, the input signal can have multiple carriers at more than 2 frequencies, and can also have multiple carriers at the same frequency (an example of the latter is MIMO streams). The carriers do not need to be at an RF carrier frequency to cause non-linear interference. An example is a strong signal that generates distortion in the analog-to-digital converter in the analog baseband domain in a receiver.
The definition of some terms used in this disclosure are as follows:                Victim signal: this generally refers to a signal of a receiver that might have non-linear interference present. This signal can also have a desired received signal, noise and other interference.        Potential offending signal: this refers to the signals that could experience the non-linear behavior of some part or object. These are generally signals transmitted from base station antennas, or other nearby transmitting devices. These signals have the potential to generate non-linear interference.        
Intermodulation interference may be detected by synthesizing the intermodulation products using the digital version of the transmitted signals. This can be done using a plurality of delays for the transmitted signals. The different intermodulation products are correlated with the actual received signal, and if the correlation data exceeds some threshold for some set of delays, then intermodulation interference has been detected. The synthesized intermodulation product is frequency shifted to align it with a real potential intermodulation product that might exist in the receiver.
With multiple transmit signals at multiple frequencies, there can be a lot of non-linear product combinations to try. The transmit signals making up a single non-linear product need to be time-aligned relative to each other before the product can be correlated with the received signal. A frequency shift must also be determined and applied prior to doing the correlation.